In a wireless communications network, it is very common for a wireless station to experience multi-cell interference when the radio signals, transmitted by a mobile wireless station (MS) or a base transceiver station (BTS) from the neighboring cells, spill over and impair the channel condition of the wireless station. Multi-cell interference limits both the capacity and coverage of a wireless communications network. The problem of multi-cell interference is especially severe when a frequency re-use factor is small.
Various techniques have been developed to reduce multi-cell interference in a wireless communications network. One of the techniques uses a different pseudorandom noise offset (PN offset) for each cell and another uses a different frequency offset for each cell. Yet another uses a directional antenna on an MS.
The PN offset technique is mainly used in a spread spectrum communications network. This technique makes interference energy spread out over a wider spectrum, instead of trying to reduce it. As a result, interference energy has the same effect as thermal noises do. When the PN offset technique is used, the degree of isolation among cells depends on the spreading gain of the wireless communications network.
The frequency offset technique assigns different frequencies to different cells. The multi-cell interference among cells is reduced because the interference signals do not appear on the same frequency as the desired signals. The disadvantage of this technique is that spectrum usage efficiency is reduced. For example, in a wireless communications network with a frequency re-use factor of 3, spectrum usage efficiency is reduced by a factor of 3.
The directional antenna technique is to have a directional antenna on an MS positioned toward the direction of arrival of desired signals and away from interference signals, which are the signals transmitted from other cells. The impact of the interference signals on a wireless station is thus reduced. However, a disadvantage of this technique is that the directional antenna is bulky and expensive; therefore, it is not practical for mobile users.
As such, what is desired is an effective and practical method for reducing the multi-cell interference in a wireless communications network in order to improve the network's capacity and coverage.